This invention pertains to a magneto-optical storage medium in which it is possible to reduce cross-talk from adjacent tracks, increase track density, and reduce the magnetic field applied during erasure.
In recent years, due to the need for managing high-volume information such as moving image data, etc., there has been a demand for a magneto-optical storage medium that has a higher level of memory storage capacity. For this reason, technical developments have advanced to the point where recording marks (magnetic domains) are micro-sized, further reducing bit density and track density. In addition, the heat distribution of the playback light beams can be utilized in a method called magnetically induced super resolution (MSR) recording/playback, which is a technique involving a recording mark is that is smaller than a spot beam diameter.
A medium to which the MSR method is applied consists of a recording layer in which a magnetic domain is recorded, as well as a multilayer that includes at least a playback layer, onto which the magnetic domain of the recording layer is transferred. When irradiating a light beam with a power level suitable for playback, a temperature distribution occurs such that some areas are heated to a higher temperature than other areas. The magnetic characteristics are adjusted so that an area with either a specified maximum temperature or a specified minimum temperature functions as a magnetic mask. As a result, this magnetic mask provides the same effect as micro-sizing the spot diameter, making it possible to playback only the desired minute marks.
Examples of this type of MSR method that have been introduced include operations conducted in response to a given position at which a mark within the light beam spot is detected, such as front aperture detection (FAD), center aperture detection (CAD), and rear aperture detection (RAD), as well as a double mask RAD method in which the areas to the front and rear of a mark to be played back are masked and the center portion is detected.
Examples of the CAD-type MSR storage medium include recording layers that are made from vertically magnetized films comprised of TbFeCo (terbium, iron, and cobalt), as well as non-magnetic layers comprised of SiN (silicon nitride). Added to the top layer is a multilayer film that contains at least a playback layer comprised of GdFeCo (gadolinium, iron, and cobalt), which at room temperature is an in-plane magnetization film within the surface of the monolayer (i.e., a single layer considered alone) that converts to a vertical magnetization film once a specified temperature is exceeded. Also, since a CAD-type medium undergoes vertical magnetization at the aperture location, which is an area in which the temperature exceeds the specified temperature of the playback layer within the spot beam during playback, it is possible for magnetization to be detected through Kerr rotation of the light beam's plane of polarization. However, since in-plane magnetization occurs along the playback layer portion that is either outside of the spot beam or outside of the aperture location, magnetization cannot be detected along this portion.
Furthermore, in the case of the CAD-type medium, magnetostatic bonding causes the magnetization direction of the playback layer to change when subjected to the magnetic field from the recording layer, and therefore it becomes necessary to make the recording layer of a TM-rich composition, which materials have a high level of magnetization (Ms). Such a material is also referred to as a TM-dominant composition. TM-rich, and TM-dominant, refer to substances in which the sub-lattice magnetization of the transition metal is higher than that of the rare earth metal.
However, the problem with this TM-rich recording layer is that a large magnetic field is required for erasure. Another problem with a TM-rich recording layer is that magnetic fields are also generated from marks that are written onto the recording layer of adjacent tracks that are not being played back. The magnetic fields from these adjacent tracks affect the track being reproduced during playback, and cross-talk occurs due to the magnetostatic bonding strength. In order to prevent these problems from occurring, it is possible to reduce the level of FeCo, which is to say that an RE-rich composition can be used. (Such a material is also referred to as an RE-dominant composition. RE-rich, and RE-dominant, refer to substances in which the sub-lattice magnetization of the rare earth metal is higher than that of the transition metal. However, since there is a low level of magnetization Ms within an RE-rich recording layer at playback temperatures, playback becomes difficult due to the fact that recording layer marks cannot be sufficiently transferred onto the playback layer.
The purpose of this invention is to present a magneto-optical storage medium that protects against the effects of cross-talk, making it possible to further reduce the level of track density, and which also makes it possible to conduct high-density recording/playback with a reduced magnetic field for erasure as well as with a favorable level of playback signal quality.
According to this invention, a magneto-optical storage medium is comprised of at least the following laminated layers, in the following order: a playback layer, a non-magnetic layer, a transfer layer, a cut-off layer, and a recording layer. The playback layer preferably displays easy in-plane magnetization characteristics within the monolayer at room temperature. In this specification, the term “monolayer” refers to a single layer in which the magnetic characteristics are measured in that single layer, without any magnetic influence from any adjacent layers. Both the transfer layer and recording layer preferably have easy magnetization characteristics in the vertical direction of the monolayers at room temperature. In addition, if the Curie temperature for the cut-off layer is designated as Tc and the Curie temperature for the recording layer is designated as Tck, then preferably Tcs<Tck. Furthermore, with a Curie temperature for the transfer layer designated as Tct, this invention is also preferably characterized by a relationship in which Tct>Tcs. In addition, with the respective film thicknesses for the transfer layer, the cut-off layer, and the recording layer given as Tt, Ts, and Tk, this invention is also preferably characterized by a relationship in which Ts<Tt and/or Ts<Tk.
The present magneto-optical storage medium is comprised of at least the following laminated layers, in the following order: a playback layer, a non-magnetic layer, a transfer layer, a cut-off layer, and a recording layer. Further, the playback layer displays easy in-plane magnetization characteristics within the monolayer at room temperature, the transfer layer and the recording layer both have easy magnetization characteristics in the vertical direction of the monolayers at room temperature, and the respective film thicknesses for the transfer layer and the cut-off layer are given as Tt and Ts, and where Tt>Ts. Furthermore, with the film thickness for the recording layer given as Tk, this magneto-optical storage medium is characterized by a relationship in which Ts<Tk.
In addition, the present magneto-optical storage medium is characterized by the fact that the playback layer is preferably a material comprised of GdFeCo, which is rich in the raw earth metal Gd at room temperature (i.e., the material is RE-rich at room temperature).
In addition, the present magneto-optical storage medium is characterized by the fact that the transfer layer is preferably a material comprised of GdDyFeCo, which is a compensating substance or is rich in its FeCo composition (TM-rich) at room temperature.
In addition, the present magneto-optical storage medium is characterized by the fact that the cut-off layer is preferably a material comprised of TbFe or TbFeCo.
In addition, the present magneto-optical storage medium is characterized by the fact that the recording layer is preferably a material comprised of rare earth-transition metals, and this substance is a compensating substance or is rich in rare earth types (RE-rich) at room temperature.
Furthermore, this magneto-optical storage medium is characterized by the fact that the cut-off recording is preferably a material comprised of TbFeCo, DyFeCo, TbDyFeCo, or GdTbDyFeCo.
Furthermore, the magneto-optical storage medium of the present invention can be used in an optical device/storage device that contains at least the following: a light emission assembly used to irradiate a light beam; a magnetic field emission assembly used to apply a magnetic field; a power control system used to set/control the power level of the light beam as it corresponds to the storage medium; and a magnetic field control system that sets/controls the direction as well as the size of the magnetic field as they correspond to each access of the storage medium.
In addition, the present invention also relates to an optical disk drive that includes a light emission assembly to irradiate a light beam upon an optical disk, a magnetic field emission assembly to apply a magnetic field to an optical disk, and a power control system to control the power of said light beam and a magnetic field control system to control the magnitude and direction of the magnetic field. The power control system and the magnetic field control system are configured such that during a data recording process, the light beam is controlled to a recording power to raise the temperature of a portion of an optical disk being recorded to a first temperature and the magnitude of the magnetic field is controlled to apply a recording magnetic field, such that magnetic domains in a recording layer of an optical disk are reoriented, without reorienting associated magnetic domains in a playback layer of an optical layer. In addition, during a data reproducing process, the light beam is controlled to a reproducing power to raise the temperature of a portion of an optical disk being recorded to a second temperature, which is lower than the first temperature, and the magnitude of the magnetic field is controlled to apply a reproducing magnetic field, which is lower than the recording magnetic field, such that magnetic domains of a playback layer of an optical disk are reoriented to coincide with associated magnetic domains in a recording layer of a magnetic disk.
In addition, the power control system and the magnetic field control system of the present optical disk drive may also be configured such that during a data erasing process, which is performed prior to said data recording process, the light beam is controlled to an erasing power, and the magnitude of the magnetic field is controlled to apply an erasing magnetic field.